Organosilicon compounds have utility in a wide range of commercial applications. For example, organosilanes are used as precursors to silicon-containing polymers, which, in turn, have numerous uses in the electronics, adhesives, and plastics industries. Organosilanes are also useful in a host of synthetic organic processes, typically as reducing agents and/or silylating agents. Other organosilicon compounds such as hydrolyzable silyl ethers (also termed alkoxysilanes) are used in the manufacture of coatings, glasses, and binders, while less reactive, lower molecular weight silyl ethers find utility as lubricant, heat-transfer, and dielectric fluids.
A well known and widely used reaction for synthesizing organosilicon compounds is the hydrosilylation of olefins, in which a silane reactant R3Si—H is added to an unsaturated carbon-carbon bond in the presence of a catalyst selected to activate the Si—H bond in the silane precursor. Hydrosilylation of olefins and carbonyl compounds has traditionally been achieved by employing low-valent late transition metal complexes as catalysts, e.g., palladium, platinum, rhodium, and iridium complexes. See Ojima et al., “Recent Advances in the Hydrosilations and Related Reactions,” in The Chemistry of Organic Silicon Compounds, vol. 2 (New York: Wiley and Sons, 1998), at pages 1687-1792. Illustrative of such catalysts are platinum supported on carbon, chloroplatinic acid, complexes of platinic chloride and unsaturated organic compounds, and compounds and complexes of rhodium, as described, for example, in U.K. Patent Application No. 1,041,237. Also see Chalk (1971), Ann. N.Y. Acad. Sci. 172(13):533-540, which describes hydrosilylation of olefins using iridium, platinum and rhodium complexes as hydrosilylation catalysts. Organosilicon compounds have also been synthesized by silylation of carbonyl compounds, including aldehydes and ketones, traditionally using Lewis acid metal complexes as catalysts.
Use of the platinum group metals as hydrosilylation catalysts is, however, problematic in several respects. Complexes of the platinum group metals are typically air- and moisture-sensitive, and, therefore, any reactions catalyzed with such complexes cannot be carried out without taking precautions to avoid air and/or water contamination. These catalysts are also expensive, precluding widespread utility on an industrial scale. In addition, use of highly Lewis acidic complexes as catalysts for the silylation of carbonyl-containing compounds precludes the use of reactants that contain Lewis basic functional groups. Furthermore, in many cases the Lewis acid catalysts are highly intolerant to the presence of water.
Accordingly, there is a need in the art for a new method of catalyzing a silylation reaction that would not be associated with the aforementioned problems. Optimally, then, the catalyst used would be air- and moisture-insensitive, significantly less expensive than the platinum-group catalysts, and tolerant of a range of functional groups. It would also be desirable if such a catalyst were useful in catalyzing stereoselective, e.g., enantioselective, silylation reactions. Furthermore, an ideal catalyst would be useful not only in silylation, but would also be useful in catalyzing other nucleophilic addition reactions wherein a nucleophilic reactant is added to an electrophilic compound containing, as an electrophilic site, an unsaturated carbon-carbon bond, a carbonyl group, a thiocarbonyl group (C═S), or an imino (C═NH) group.